I. Field of the Invention
The present invention relates to broadcast communications, otherwise known as point-to-multipoint or group communications, in a wireline or a wireless communication system. More particularly, the present invention relates to a system and method for providing frame re-transmission in such a broadcast communication system.
II. Description of the Related Art
The field of wireless communications has many applications including cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term “cellular” systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, such as IS-95A, IS-95B (often referred to collectively as IS-95), ANSI J-STD-008, IS-99, IS-2000, IS-657,IS-707, and others, are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies. Generally, information transmitted in such systems is formatted into discreet packets, otherwise known as data packets or data frames, or simply frames. To increase the likelihood that a frame will be successfully transmitted during transmission various re-transmission schemes have been developed. For example, a radio link protocol (RLP) has been developed to perform frame re-transmissions when one or more frames are not successfully received. The RLP protocol controls how and when frames are re-transmitted from a transmit system to a receiver.
In order to determine which frames have not been received successfully using RLP, an eight-bit sequence number is included as a frame header in each frame transmitted. The sequence number is incremented for each frame from 0 to 256 and then reset back to zero. An unsuccessfully received frame is detected when a frame with an out-of-order sequence number is received, or an error is detected using CRC checksum information or other error detection methods. Once an unsuccessfully received frame is detected, the receiver transmits a negative-acknowledgment message (NAK) to the transmit system that includes the sequence number of the frames that were not received correctly. The transmit system then re-transmits the frame including the sequence number as originally transmitted. If the re-transmitted frame is still not received successfully, a second re-transmission request is sent to the transmit system, this time requesting that the frame be transmitted twice. If the frame is still not received successfully, a third retransmission request is sent to the transmit system, this time requesting that the frame be transmitted three times. If the frame is still not received successfully after the third retransmission request, no further retransmissions are requested, and the frame is ignored at the receiver for use in reconstructing the original data.
The frame re-transmission scheme just described is an example of a scheme used for point-to-point communications between a transmit system and a receiver. Such a re-transmission system is impractical in a broadcast communication system because of the potential for an almost unlimited number of re-transmission requests from the various receivers. Specifically, for any given frame, the probability of a frame being received in error (or not at all) is much higher in a broadcast transmission system than in a point-to-point broadcast system. As the number of receivers increase, the probability of a retransmission being needed increases dramatically. Also, as the number of re-transmissions increase, the delay, or latency, of transmitting new information increases to a point at which an application using the information at a receiver is noticeably impacted. For example, delays of more than a few hundred milliseconds in speech can result in unacceptable voice quality at a receiver. Furthermore, this retransmission scheme also depends on the existence of a feedback channel. That is, there has to be two-way communication between the transmitter and the receiver for the scheme to function properly. In some systems, such a two-way link might not be available.
What is needed is a method and apparatus for frame re-transmission in a broadcast communication system which avoids the problems of latency.